The diagnostic process:
Tufail popped his car into us for engine light being on, but driving perfectly.
Most people think you plug a car in and it tells you what is wrong, I wish!!!
Step 1- Confirm the complaint.
Sounds silly but we have to see for ourselves. So I had a look and yes light was on, took her for a spin and indeed driving perfectly.
Step 2 - Code reading
Plug in a diagnostic machine and scan the car for fault codes. In this instance it had recorded a drop in signal from the diesel temperature sensor. This tends to be the point where some out there will bang in a new sensor, this tends to be the places that don't charge for diagnostic testing which is fair enough as step 2 is not diagnosis it is basic code reading anyone can buy a code reader off ebay for about £20.
Step 3 - Visual & tactile inspection
Very obvious stuff but I had a look and feel around under the bonnet to ensure nothing was amiss or hanging off. All good so far and temperature sensor where it should be and plugged in.
Step 4 - Diagnostic testing
At this point I took a diagnostic machine and read the live data from the car, so I can see the information the engine ecu is seeing. I could see that the computer was getting a signal from the sensor as the diesel warmed up, but it was erratic and dropping down very low intermittently. I could then disconnect the sensor and fit a variable resistor in place, this enabled me to make the computer read whatever temperature I like and monitor if it was getting a good signal. Sure enough the temperature was reading, but still very erratic at points.
Whilst this indicates that it is a wiring or ECU fault the sensor is not out of the woods as we need to get the wiring or ECU working correctly before we can test the sensor further.
However this does mean we are not calling the customer saying 'it just needs a sensor, that's £xx' then having to call later to say we need to spend more of your money as it's not fixed.
An inspection of the connector from the loom to the sensor with a magnifying glass showed excessive corrosion on both terminals and one had splayed out over time giving poor contact. I bypassed the connector by back probing it and the test with the resistor was carried out again.
Now we have a constant reading doing exactly as it should.
With the makeshift wiring linking the loom to the sensor we can now monitor the quality of the signal from that. For the first five minutes of running it was perfect, but then took a couple of dives. Now we could confirm the sensor was also past its best and building resistance as it got hot.
Step 5 - Repair
Clean up and realignment of the terminals and a new sensor fitted
Step 6 - Confirmation of repair
With the car all back together as it should be it was run up to temperature whilst monitoring the live data, readings were spot on. Then a road test carried out to ensure light did not return and then one last code scan to see if anything had come back.
Done, it's that easy